The purpose of the proposed research is to test a theory of age differences in processing resources for semantic and episodic memory. Salthouse has hypothesized that processing speed is a processing resource. Allen has proposed that neural noise as measured by entropy provides an even more basic measure of age differences in processing resources. Specifically, increased neural noise reduces the available processing resources for cognitive processes. The goal of the present project is to determine why differential age differences occur for episodic and semantic memory tasks across peripheral and central processing stages by examining whether practice and/or task type affects these age differences. The present proposal contains eight experiments that are designed to determine how entropy (a measure of neural noise), processing speed, and processing accuracy are related to age differences in processing resources across different processing stages (peripheral vs. central) and tasks (episodic vs. semantic memory). Experiments 1-2 will examine the time-accuracy functions of healthy young and older adults on lexical decision and memory search tasks that vary exposure duration. Accuracy data will be used to derive entropy values. Experiments 3-4 will use Pashler's psychological refractory period paradigm to examine age differences in processing resources as the stimulus onset asynchrony is varied between two tasks (Experiment 3 will use dot location followed by a lexical decision task: Experiment 4 will use dot location followed by lexical decision or memory search tasks). Experiment 4 will also examine the effect of practice on age differences in processing resources by using four sessions per participant. Experiments 5-8 will consist of four within subject experiments in which the same stimuli (words or numbers) are used for both semantic and episodic memory tasks. Experiments 5-8 will rule out stimulus confounds that were present in earlier studies on this topic.